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Large Capacitor to High Pass the Woofer in speakers?

D

DannyBad

New Member
Does anybody do this? Something like a 660uf Capcitor to cut below 60Hz on my small 4 ohm woofer in desktop speakers. Can you do this then have a second order low pass filter (Inductor in series and capacitor in parallel)?
In my experience, cutting out the low end from these speakers can really clean everything up and the sub can handle all of those frequencies.
 
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I have a pair of Realistic Optimus 7 that have two 330 uF caps in series to reduce the bass response. That is the same as a single 660 uF cap in series with the woofer. There is a switch to bypass the caps for normal response.

A high value cap in series with the woofer acts like a first order high pass filter. There will be a slight drop: 6 dB per octave.

But if you really wanted to cut off the lower frequencies to the woofer to blend with a subwoofer, a second or third order filter would be much better. That would give you 12 or 18 dB per octave.
 
RTally said:
I have a pair of Realistic Optimus 7 that have two 330 uF caps in series to reduce the bass response. That is the same as a single 660 uF cap in series with the woofer. There is a switch to bypass the caps for normal response.

A high value cap in series with the woofer acts like a first order high pass filter. There will be a slight drop: 6 dB per octave.

But if you really wanted to cut off the lower frequencies to the woofer to blend with a subwoofer, a second or third order filter would be much better. That would give you 12 or 18 dB per octave.
Click to expand...
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I think you've done it in reverse. Two 330 uF caps in SERIES (to each other) would total 165 uF. They'd be 660 in PARALLEL(to each other).
 
DannyBad said:
Does anybody do this? Something like a 660uf Capcitor to cut below 60Hz on my small 4 ohm woofer in desktop speakers. Can you do this then have a second order low pass filter (Inductor in series and capacitor in parallel)?
In my experience, cutting out the low end from these speakers can really clean everything up and the sub can handle all of those frequencies.
Click to expand...
There's also filtering at the preamplified level where a filter is inserted between the preamp and the amp. It's a lot easier to do it at that point than it is after the amplfication.
 
Because the speaker impedance varies significantly with frequency on the low end, adding a series cap is a crap shoot, especially if the speaker is ported.
 
DannyBad said:
Does anybody do this? Something like a 660uf Capcitor to cut below 60Hz on my small 4 ohm woofer in desktop speakers. Can you do this then have a second order low pass filter (Inductor in series and capacitor in parallel)?
In my experience, cutting out the low end from these speakers can really clean everything up and the sub can handle all of those frequencies.
Click to expand...
It would be possible, but not very elegant.
Porkloin said:
There's also filtering at the preamplified level where a filter is inserted between the preamp and the amp. It's a lot easier to do it at that point than it is after the amplfication.
Click to expand...
Yes, this is preferable in my opinion.
Porkloin said:
.
I think you've done it in reverse. Two 330 uF caps in SERIES (to each other) would total 165 uF. They'd be 660 in PARALLEL(to each other).
Click to expand...
That is correct - 2 x 330uF in series equals 165uF.
 
W9TR said:
Because the speaker impedance varies significantly with frequency on the low end, adding a series cap is a crap shoot, especially if the speaker is ported.
Click to expand...
That makes a lot of sense to me. I am using two 4" drivers and will be porting them so I will leave the low end alone.
 
Porkloin said:
There's also filtering at the preamplified level where a filter is inserted between the preamp and the amp. It's a lot easier to do it at that point than it is after the amplfication.
Click to expand...
For sure an active crossover is highly preferable in this case. I was just wondering if anybody had ever don it passively and what the effects might be.
 
nerdorama said:
Sometimes a large cap in series causes a resonance in the low bass which extends the low end response. Infinity used this method in a number of their large speakers in days past.

Just a for instance: http://www.infinity-classics.de/technik/manuals/Kappa_9_technical_sheet.pdf

John
Click to expand...

Correct.

This effect is more common than most people think. In fact, with the rising impedance of the woofer nearing resonance- the cap will have very little effect, in REDUCING output there- but it will act to reduce the damping (by increased series impedance with the speaker). This can actually make the bass, at resonance, LOUDER, unless the cap is SO small that it has SO much impedance at those frequencies, that it just doesn't pass any significant energy at all.

In short- if you're going to do this, be prepared to play around with it- and for the cap value to wind being unpredictably but significantly smaller than textbook value..

Regards,
Gordon.
 
GordonW said:
Correct.

This effect is more common than most people think. In fact, with the rising impedance of the woofer nearing resonance- the cap will have very little effect, in REDUCING output there- but it will act to reduce the damping (by increased series impedance with the speaker). This can actually make the bass, at resonance, LOUDER, unless the cap is SO small that it has SO much impedance at those frequencies, that it just doesn't pass any significant energy at all.

In short- if you're going to do this, be prepared to play around with it- and for the cap value to wind being unpredictably but significantly smaller than textbook value..

Regards,
Gordon.
Click to expand...
That makes total sense and I think in this situation (two 4" woofers in a ported enclosure) I am only going to be asking for problems. A much better solution would be to have an external DSP do it. The result will be much more accurate.
 
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